(1) Field of the Invention
This invention relates to a recording material. More particularly, the invention relates to a pressure-sensitive recording material which comprises a sheet material having thereon a layer of microcapsules containing an electron donating dye-precursor and a specific solvent mixture for the dye-precursor. The specific solvent mixture consists of 1-methyl-3-phenylindane and at least one of diarylalkanes.
(2) Description of the Prior Art
Several kinds of pressure-sensitive recording materials are hitherto well known. For example, to one sheet of paper are applied microcapsules containing a solution of an electron donating colorless chromogenic substance (hereinafter referred to as "dye-precursor"), and to another sheet of paper is applied an electron accepting substance such as clay or polymeric material (hereinafter referred to as "color developer") which produces a color upon reacting with the dye-precursor. When the recording materials are used, the treated surfaces of the above sheets are put together face to face and local pressure is applied to the paired papers by handwriting or typewriting, thereby obtaining desired duplicate impressions.
The recording mechanism in the pressure-sensitive recording material of the type described above is such that the microcapsules are ruptured by the pressure of handwriting or the impact of typewriting to release the dye-precursor solution from the microcapsules. The solution containing a dye-precursor comes into contact with the color developer on the opposing surface of the other sheet of paper to produce a color.
In another type of known recording material, the microcapsule layer is applied to one side of a sheet of paper as an inner layer, and a color developer is then applied as an outer layer over the microcapsule layer. Similarly, when it is used, the microcapsules on this recording material are ruptured by the pressure of handwriting or typewriting and the dye-precursor solution is released from the microcapsules, which solution comes into contact with the color developer in the outer layer to produce a color.
The dye-precursor solution that is used for the recording material of this kind is a solution of an electron donating dye-precursor in one or more kinds of hydrophobic solvents. The hydrophobic solvents employed here are required to have the properties such as innoxiousness, absence of disagreeable odor, colorlessness or quite light color, non-volatility, good dissolving property (solvent power) for dye-precursors and good stability when a dye-precursor is dissolved therein. It is further required that, in the preparation of microcapsules, a very fine and stable dispersion of the solution can be produced; microcapsule membranes can be formed around the fine particles of the dispersion; the obtained microcapsules have good storage stability; the membranes of microcapsules are uniform and of desired thickness; the solvent does not inhibit the color-producing reaction between a dye-precursor and a color developer with a high reaction rate; when the base paper is coated with a polymeric material as a color developer, the solvent dissolves the polymeric material to cause close contact with the dye-precursor; duplicate impressions are clear without runs; and that duplicate impressions can be maintained as they stand for a long period of time without changing.
In the prior art, polychlorinated biphenyl was widely used as the solvent for producing the microcapsules of these pressure-sensitive recording materials. The polychlorinated biphenyl has indeed several excellent properties as the solvent of this kind; however, it has a serious disadvantage in that it is quite toxic and its accumulation in the human body can cause several disorders. Therefore, when the recording material having a microcapsule layer containing the polychlorinated biphenyl is handled in the production process and used in recording, it may cause serious health problems. Accordingly, the polychlorinated biphenyl is no longer being used. As the solvents to replace polychlorinated biphenyl, there are proposed and used, for example, diarylalkanes such as 1-phenyl-1-xylylethane and 1-phenyl-1-(ethylphenyl)ethane, alkylnaphthalene and alkylbiphenyl (U.S. Pat. No. 3,936,566--A. Sato et al.; U.S. Pat. No. 3,836,383--M. Kiritani et al.) However, these solvents are not satisfactory in view of their dissolving property for dye-precursors, especially for black dye-precursors. Also proposed is the use of alkylarylindane obtained as a dimer of styrene or the like (British Pat. No. 1,406,107--Monsanto Co.; Japanese Laid-Open Patent Publication No. 55-63292--Asahi Dow). However, in these solvents, the color developing rate and developed color density are insufficient.